1. Technological Field
The present invention relates to an optical add-drop device suitable for use in wavelength-division multiplexed optical communications systems, wavelength-division multiplexed optical communications networks, and other optical communications systems.
2. Background Technology
In recent years optical add-drop devices for splitting or inserting only certain wavelengths have been proposed for the optical undersea branching devices used in the optical submarine cable systems using wavelength-division multiplexed optical communications systems (for example, K. P. Jones, et al., "Optical wavelength add-drop multiplexer in installed submarine WDM network," Electronics Letters, Vol. 31, No. 24, pp. 2117-2118, 1995).
An example of the structure of such a conventional optical add-drop device is depicted in FIG. 5. In the drawing, 50 is an input optical line, 51 and 52 are optical lines, 53 is a branching optical line, 54 is an inserting optical line, and 55 is an output optical line. In addition, 60 and 61 are optical circulators, 70 is an optical fiber grating, 80 through 82 are terminals for inputting light signals to and outputting them from the optical circulator 60, and 83 through 85 are terminals for inputting optical signals to and outputting them from the optical circulator 61.
Here, the aforementioned optical fiber grating 70 is such that the composition of a silica optical fiber having a germanium-doped core is varied, as is the retractive index, by means of an interference pattern formed by irradiating the fiber with strong ultraviolet light from two directions, and a function of the grating is to reflect light signals whose wavelengths correspond to the pattern pitch. In addition, the aforementioned optical circulators 60 and 61 operate in such a way that signals inputted from the terminals are rotated in the directions shown by the arrows in the drawing. For example the optical circulator 60 operates in such a way that light signals inputted to the terminal 80 are outputted from the terminal 81, light signals inputted to the terminal 81 are outputted from the terminal 82, and light signals inputted to the terminal 82 are outputted from the terminal 80.
In an optical add-drop device with such a structure, wavelength-division multiplexed signal light inputted to the input optical line 50 is inputted to the input terminal 80 of the optical circulator 60 and, as described above, is outputted to the optical line 51 via the terminal 81. The wavelength-division multiplexed signal light outputted to the optical line 51 is inputted to the optical fiber grating 70. Of the inputted wavelength-division multiplexed signal light, only signal light of prescribed wavelength is reflected in the direction of the optical line 51 due to the filtering function of the optical fiber grating 70, and signal light having other wavelengths is transmitted to the optical line 52.
The signals of the prescribed wavelength that have been reflected by the aforementioned optical fiber grating 70 and sent out to the optical line 51 are inputted to the terminal 81 of the optical circulator 60, outputted from the terminal 82, and launched into the branching optical line 53. Meanwhile, signals of other wavelengths that have been transmitted through the aforementioned optical fiber grating 70 are inputted to the terminal 83 of the optical circulator 61 via the optical line 52, outputted from the terminal 84, and launched into the output optical line 55.
Meanwhile, when signal light having the same prescribed wavelength as the signal light outputted to the aforementioned branching optical line 53 is inputted from the inserting optical line 54, this signal light is inputted from the terminal 85 to the aforementioned optical circulator 61 and outputted from the terminal 83. This signal light is propagated through the optical line 52 and inputted to the optical fiber grating 70, and this signal light is reflected into the optical line 52 by the action of the optical fiber grating 70 because, as described above, the wavelength of this signal light is the same as the prescribed wavelength reflected by the optical fiber grating 70. The reflected signal light is again inputted to the optical circulator 61 through the terminal 83 and is launched to the output optical line 55 via the terminal 84.
Thus, in a conventional optical add-drop device, only the signal light that has the prescribed wavelength determined by the optical fiber grating 70 is outputted to the branching optical line 53 out of the wavelength-division multiplexed signal light inputted from the input optical line 50, and signal light that has the same wavelength as the signal light outputted to the branching optical line 53 can be inserted from the inserting optical line 54, making it possible to the reuse the light wavelength.
However, the conventional optical add-drop device described above has the following properties 1 through 3.
(1) The wavelength of signal light involved in branching or insertion is fixed by the wavelength of the optical fiber grating 70 and cannot be changed. PA1 (2) The number of signal lights involved in branching or insertion is fixed at one, making it impossible to branch or insert a plurality of signal lights. PA1 (3) The wavelength of signal light outputted by a branching optical line and the wavelength of signal light inputted from an inserting optical line must be the same.
It is therefore difficult to change the number or the wavelength of signal lights involved in branching or insertion once the system has been built, making it impossible to create a flexible network structure.
In view of the above, an object of the present invention is to provide an optical add-drop device which allows an arbitrary number of signal lights having arbitrary wavelengths to be branched or inserted and which does not require that the wavelength of signal light outputted from a branching optical line be the same as the wavelength of signal light inputted from an inserting optical line.